Data backup is a standard part of all large-scale computer data storage systems (and most small systems, as well). Data written to a primary storage medium, such as a local disk, are copied to a backup medium, such as another disk or a tape, which can then be used for recovery in case a disaster causes the data on the primary medium to be lost. Periodic backup of this sort is adequate for many storage systems. In mission-critical systems, however, it is frequently necessary that stored data be backed up immediately, as soon as the data are written to the primary medium. Therefore, large storage systems for critical storage applications typically provide a variety of copy services that can be called for immediate data backup while the application is running.
For example, the IBM Enterprise Storage Server (ESS), sold by IBM Corporation of Armonk, N.Y., supports several hardware-assisted copy functions to provide data mirroring for disaster recovery. These functions are described by Kulzer et al., in a publication entitled IBM Enterprise Storage Server (IBM International Technical Support Organization, San Jose, Calif., 1999), which is incorporated herein by reference. A number of different copy service functions of the ESS are described in Chapter 6 of this publication. These functions include:                Concurrent copy, which creates an instant (time zero) copy of a source volume or data set, while allowing the source to be modified in the meanwhile. The copy process is logically complete when a System Data Mover (SDM) component of the host operating system has figured out what data to copy. When an update to the source is to be performed before the data have been physically copied to the target, the original source data are first copied to a “sidefile” in cache memory. Up to 64 copying sessions can be active at one time.        Peer-to-peer remote copy (PPRC), in which a synchronous mirror copy of a source volume on a primary storage subsystem is created on a secondary storage subsystem. The primary and secondary storage subsystems typically belong to separate ESS systems, at mutually-remote locations. PPRC is implemented using a direct connection between the primary and secondary subsystems. When an application on a host processor writes to a PPRC volume on the primary subsystem, the corresponding data updates are entered into cache memory and non-volatile storage at the primary subsystem. The primary subsystem then sends the updates over the link to the secondary subsystem. When the secondary subsystem has placed the data in its own cache and non-volatile storage, it acknowledges receipt of the data, and the primary subsystem then signals the application that the write operation is complete.        Extended remote copy (XRC), like PPRC, creates a mirror copy on a secondary storage subsystem of a source volume on a primary storage subsystem. XRC, however, is an asynchronous process carried out by the SDM component of the host operating system. Applications on the host that write data to a source XRC volume receive notification from the primary subsystem that the write is complete as soon as the data have been secured in cache and non-volatile storage on the primary subsystem. In a separate operation, the SDM reads out the updates of the data from the cache and sends them to the secondary subsystem for mirror storage.        
In a typical large-scale storage system, serving many different host applications, there are likely to be multiple instances of all of these different backup operations going on at one time. These operations can consume large amounts of system resources, such as processing capacity and cache memory. There is therefore a need for tools that can be used to manage the various backup operations in an efficient manner that conserves cache memory.